Interaction of shale gas recovery and moisture transport in post two-phase flowback stage

In the post two-phase flowback stage of shale gas recovery, the residual water in fractures no longer forms a stable continuous flow but mitigates through moisture transport. How this residual water affects gas production is still not clear. This paper studies the motion of this residual water during shale gas recovery. Following three aspects are mainly studied: First, the residual water forms a water film on the surface of fractures and makes Darcy flow in fractures no longer applicable. A new empirical formula for threshold pressure gradient is proposed and directly related to residual water saturation. Second, moisture transports through diffusion, convection and evaporation reaction. Especially, the evaporation of water film depends on surrounding humidity, water activity, and gas flow rate. This evolution of water film is a dynamic equilibrium process and impacts fracture permeability. Third, residual water in matrix interacts with clay minerals to form bonded water. This part of water modifies the Langmuir isotherm adsorption and changes the adsorption capacity of shale gas. Thus the moisture transport significantly affects the shale gas recovery, which should be carefully considered.